Objective assessment of the fusion frequency in functional electrical stimulation using the fast Fourier transform.

In functional electrical stimulation (FES) the dynamics of tetanic muscle contractions is often described by the fusion frequency (FF), as determined by palpation: contractions elicited by stimulation frequencies above the FF appear smooth. To contribute to a more objective assessment of this important FES parameter, we have developed a dedicated signal analysis method based on fast Fourier transformation (FFT). The ripple to peak ratio (R(rpFFT)) - the relation between ripple amplitude and peak force value of a recorded tetanic muscle force in relation to the applied stimulation frequency - was determined automatically by analysing a 0.2-s interval in the steady state of a stimulation burst. The method was tested on simulated data and on force recordings from isolated tibialis anterior muscles of six rabbits. The results were compared to manual estimates. The robustness of the method was tested by adding noise and hum. Simulated noise at 100% of the ripple force increased R(rpFFT) by 4%. Hum at 20 Hz away from the stimulation frequency caused changes of less than 0.5%. The results of the automated analysis of recorded signals matched the manual estimates sufficiently well, especially for stimulation frequencies near or above FF. R(rpFFT) therefore seems suitable for automated, objective and robust assessment of the ripple and the FF of electrically stimulated muscle.     

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions.

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.     

Length-dependent twitch contractile characteristics of skeletal muscle

The length dependence of force development of mammalian skeletal muscles was evaluated during twitch, double-pulse, and tetanic contractions, and the relation between muscle length and the time-dependent characteristics of twitch and double-pulse contractions were determined. In situ isometric contractions of the rat gastrocnemius muscle were analyzed at seven different lengths, based on a reference length at which the maximal response to double-pulse contractions occurred (Lopt-2P). Twitch and double-pulse contractions were analyzed for developed tension (DT), contraction time (tC), average rate of force development (DT-tC(-1)), half-relaxation time (t50%R), peak rate of relaxation (DT x dtmin(-1)), and 90%-relaxation time (t90%R). Considering the length at which maximal tetanic DT occurred to be the optimal length (Lo-TET), the peak DT for twitch contractions and double-pulse contractions was observed at Lo-TET + 0.75 mm (p < 0.05) and Lo-TET + 0.1 mm (p > 0.05), respectively. When measured at the length for which maximal twitch and double-pulse contractions were obtained, tetanic DT was 95.2 +/- 3 and 99.0 +/- 2% of the maximal value, respectively. These observations suggest that double-pulse contractions are more suitable for setting length for experimental studies than twitch contractions. Twitch and double-pulse contraction tC were 15.53 +/- 1.14 and 25.0 +/- 0.6 ms, respectively, at Lopt-2P, and increased above Lopt-2P and decreased below Lopt-2P. Twitch t50%R was 12.18 +/- 0.90 ms at Lopt-2P, and increased above Lopt-2P and below Lopt-2P. Corresponding changes for double-pulse contractions were greater. Stretching the muscle leads to slower twitch contractions and double-pulse contractions, but the mechanisms of this change in time course remain unclear.     

Force depression in human quadriceps femoris following voluntary shortening contractions.

The purpose of this study was to investigate whether the isometric muscle force, redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle, is smaller than the purely isometric muscle force at the corresponding length. Isometric knee extensor moments, surface electromyographic (EMG) signals of quadriceps femoris, and interpolated twitch moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle shortening for voluntary contractions. We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM.     

Peak picking multidimensional NMR spectra with the contour geometry based algorithm CYPICK

The automated identification of signals in multidimensional NMR spectra is a challenging task, complicated by signal overlap, noise, and spectral artifacts, for which no universally accepted method is available. Here, we present a new peak picking algorithm, CYPICK, that follows, as far as possible, the manual approach taken by a spectroscopist who analyzes peak patterns in contour plots of the spectrum, but is fully automated. Human visual inspection is replaced by the evaluation of geometric criteria applied to contour lines, such as local extremality, approximate circularity (after appropriate scaling of the spectrum axes), and convexity. The performance of CYPICK was evaluated for a variety of spectra from different proteins by systematic comparison with peak lists obtained by other, manual or automated, peak picking methods, as well as by analyzing the results of automated chemical shift assignment and structure calculation based on input peak lists from CYPICK. The results show that CYPICK yielded peak lists that compare in most cases favorably to those obtained by other automated peak pickers with respect to the criteria of finding a maximal number of real signals, a minimal number of artifact peaks, and maximal correctness of the chemical shift assignments and the three-dimensional structure obtained by fully automated assignment and structure calculation.     

Aging alters contractile properties and fiber morphology in pigeon skeletal muscle

In this study, we tested the hypothesis that skeletal muscle from pigeons would display age-related alterations in isometric force and contractile parameters as well as a shift of the single muscle fiber cross-sectional area (CSA) distribution toward smaller fiber sizes. Maximal force output, twitch contraction durations and the force–frequency relationship were determined in tensor propatagialis pars biceps muscle from young 3-year-old pigeons, middle-aged 18-year-old pigeons, and aged 30-year-old pigeons. The fiber CSA distribution was determined by planimetry from muscle sections stained with hematoxylin and eosin. Maximal force output of twitch and tetanic contractions was greatest in muscles from young pigeons, while the time to peak force of twitch contractions was longest in muscles from aged pigeons. There were no changes in the force–frequency relationship between the age groups. Interestingly, the fiber CSA distribution in aged muscles revealed a greater number of larger sized muscle fibers, which was verified visually in histological images. Middle-aged and aged muscles also displayed a greater amount of slow myosin containing muscle fibers. These data demonstrate that muscles from middle-aged and aged pigeons are susceptible to alterations in contractile properties that are consistent with aging, including lower force production and longer contraction durations. These functional changes were supported by the appearance of slow myosin containing muscle fibers in muscles from middle-aged and aged pigeons. Therefore, the pigeon may represent an appropriate animal model for the study of aging-related alterations in skeletal muscle function and structure.     

Opposite effects of cooling on twitch contractions of skeletal muscle isolated from tropical toads (Leptodactylidae) and northern frogs (Ranidae)

Cooling increases the twitch force of frog skeletal muscle (Rana temporaria; Rana pipiens), but decreases the twitch force of tropical toad muscle (Leptodactylus insularis). Action potentials and intramembranous charge movement in frog and toad fibers were slowed identically by cooling. Cooling increased the integral of twitch Ca²⁺ detected by aequorin in frog fibers (1.4-fold), while also decreasing the peak and slowing the rate of decay. Conversely, cooling decreased the integral (0.6-fold) and the peak of twitch Ca²⁺ in toad fibers, without affecting the rate of decay. The difference in entire Ca²⁺ transients may account for cold-induced twitch potentiation in frogs and twitch paralysis in toads. In sustained contractions of toad fibers, cooling markedly decreased maximum force caused by: (i) tetanic stimulation, (ii) two-microelectrode voltage clamp steps, (iii) high [K⁺], or (iv) caffeine. Maximum force in sustained contractions was decreased moderately by cooling frog fibers. Rapid rewarming and simultaneous removal of high [K⁺] or caffeine during a sustained contraction, caused toad muscle force to rise towards the value corresponding to the warm temperature. This did not occur after removing high [K⁺] or caffeine from toad fibers kept in the cold. Transmission electron micrographs showed no relevant structural differences. Parvalbumins are thought to promote relaxation of frog muscle in the cold. The unique parvalbumin isoforms in toad muscle apparently lack this property. Accepted: 27 August 1998     

Effects of high-pressure acclimatization on silver eel (Anguilla anguilla, L.) slow muscle contraction

As the spawning migration of the eel is supposed to correspond to a long swimming activity at depth, patterns of slow red muscle contraction have been investigated in European silver eel (Anguilla anguilla L.) exposed for 3 weeks to 10.1 MPa hydrostatic pressure. The results show that pressure-acclimated eels (male and female) show a three-fold decrease in maximum isometric stress of twitch and tetanic contractions while time to peak force, time from peak force to 90% relaxation and ratio of twitch tension to tetanic tension remain unchanged. The observed modifications in slow red muscle mechanical properties do not impede the spawning migration of the eel and are possibly partially compensated by an improvement in the efficiency of oxidative phosphorylation. Effects of changes in membrane fluidity are also discussed.     

Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.     

Filtering the surface EMG signal: Movement artifact and baseline noise contamination

The surface electromyographic (sEMG) signal that originates in the muscle is inevitably contaminated by various noise signals or artifacts that originate at the skin-electrode interface, in the electronics that amplifies the signals, and in external sources. Modern technology is substantially immune to some of these noises, but not to the baseline noise and the movement artifact noise. These noise sources have frequency spectra that contaminate the low-frequency part of the sEMG frequency spectrum. There are many factors which must be taken into consideration when determining the appropriate filter specifications to remove these artifacts; they include the muscle tested and type of contraction, the sensor configuration, and specific noise source. The band-pass determination is always a compromise between (a) reducing noise and artifact contamination, and (b) preserving the desired information from the sEMG signal. This study was designed to investigate the effects of mechanical perturbations and noise that are typically encountered during sEMG recordings in clinical and related applications. The analysis established the relationship between the attenuation rates of the movement artifact and the sEMG signal as a function of the filter band pass. When this relationship is combined with other considerations related to the informational content of the signal, the signal distortion of filters, and the kinds of artifacts evaluated in this study, a Butterworth filter with a corner frequency of 20 Hz and a slope of 12 dB/oct is recommended for general use. The results of this study are relevant to biomechanical and clinical applications where the measurements of body dynamics and kinematics may include artifact sources.     

Residual force enhancement following eccentric induced muscle damage

During lengthening of an activated skeletal muscle, the force maintained following the stretch is greater than the isometric force at the same muscle length. This is termed residual force enhancement (RFE), but it is unknown how muscle damage following repeated eccentric contractions affects RFE. Using the dorsiflexors, we hypothesised muscle damage will impair the force generating sarcomeric structures leading to a reduction in RFE. Following reference maximal voluntary isometric contractions (MVC) in 8 young men (26.5±2.8y) a stretch was performed at 30°/s over a 30° ankle excursion ending at the same muscle length as the reference MVCs (30° plantar flexion). Surface electromyography (EMG) of the tibialis anterior and soleus muscles was recorded during all tasks. The damage protocol involved 4 sets of 25 isokinetic (30°/s) lengthening contractions. The same measures were collected at baseline and immediately post lengthening contractions, and for up to 10min recovery. Following the lengthening contraction task, there was a 30.3±6.4% decrease in eccentric torque (P<0.05) and 36.2±9.7% decrease in MVC (P<0.05) compared to baseline. Voluntary activation using twitch interpolation and RMS EMG amplitude of the tibialis anterior remained near maximal without increased coactivation for MVC. Contrary to our hypothesis, RFE increased (～100-250%) following muscle damage (P<0.05). It appears stretch provided a mechanical strategy for enhanced muscle function compared to isometric actions succeeding damage. Thus, active force of cross-bridges is decreased because of impaired excitation-contraction coupling but force generated during stretch remains intact because force contribution from stretched sarcomeric structures is less impaired.     

Automatic physiological waveform processing for FMRI noise correction and analysis

Functional MRI resting state and connectivity studies of brain focus on neural fluctuations at low frequencies which share power with physiological fluctuations originating from lung and heart. Due to the lack of automated software to process physiological signals collected at high magnetic fields, a gap exists in the processing pathway between the acquisition of physiological data and its use in fMRI software for both physiological noise correction and functional analyses of brain activation and connectivity. To fill this gap, we developed an open source, physiological signal processing program, called PhysioNoise, in the python language. We tested its automated processing algorithms and dynamic signal visualization on resting monkey cardiac and respiratory waveforms. PhysioNoise consistently identifies physiological fluctuations for fMRI noise correction and also generates covariates for subsequent analyses of brain activation and connectivity.     

Reproducible measurement of voluntary activation of human elbow flexors with motor cortical stimulation.

Voluntary activation of muscle is commonly quantified by comparison of the extra force added by motor nerve stimulation during a contraction [superimposed twitch (SIT)] with that produced at rest by the same stimulus (resting twitch). An inability to achieve 100% voluntary activation implies that failure to produce maximal force output from the muscle must have occurred at a site at or above the level of the motoneurons. We have used cortical stimulation to quantify voluntary activation. Here, incomplete activation implies a failure at or above the level of motor cortical output. With cortical stimulation, it is inappropriate to compare extra force evoked during a contraction with the twitch evoked in resting muscle because motor cortical and spinal cord excitability both increase with activity. However, an appropriate "resting twitch" can be estimated. We previously estimated its amplitude by extrapolation of the linear relation between SIT amplitude and voluntary torque calculated from 35 contractions of >50% maximum (Todd G, Taylor JL, and Gandevia SC. J Physiol 551: 661-671, 2003). In this study, we improved the utility of this method to enable evaluation of voluntary activation when it may be changing over time, such as during the development of fatigue, or in patients who may be unable to perform large numbers of contractions. We have reduced the number of contractions required to only three. Estimation of the resting twitch from three contractions was reliable over time with low variability. Furthermore, its reliability and variability were similar to the resting twitch estimated from 30 contractions and to that evoked by conventional motor nerve stimulation.     

K+-induced twitch potentiation is not due to longer action potential

The objective of this study was to determine whether an increased duration of the action potential contributes to the K+-induced twitch potentiation at 37 degrees C. Twitch contractions were elicited by field stimulation, and action potentials were measured with conventional microelectrodes. For mouse extensor digitorum longus (EDL) muscle, twitch force was greater at 7-13 mM K+ than at 4.7 mM (control). For soleus muscle, twitch force potentiation was observed between 7 and 11 mM K+. Time to peak and half-relaxation time were not affected by the increase in extracellular K+ concentration in EDL muscle, whereas both parameters became significantly longer in soleus muscle. Decrease in overshoot and prolongation of the action potential duration observed at 9 and 11 mM K+ were mimicked when muscles were respectively exposed to 25 and 50 nM tetrodotoxin (TTX; used to partially block Na+ channels). Despite similar action potentials, twitch force was not potentiated by TTX. It is therefore suggested that the K+-induced potentiation of the twitch in EDL muscle is not due to a prolongation of the action potential and contraction time, whereas a longer contraction, especially the relaxation phase, may contribute to the potentiation in soleus muscle.     

Effects of speed and distance of muscle shortening on force depression during voluntary contractions

The purpose of this study was to determine the influence of speed and distance of muscle shortening on the amount of force depression for voluntary contractions. Two experimental tests were performed. In the first test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions over the range 25-50 degrees. In the second test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions at two speeds: 20 and 240 degrees /s. Knee extensor moments, surface electromyographical (EMG) signals of quadriceps femoris, and interpolated twitch moments were measured during all contractions and were compared with the corresponding values obtained during purely isometric contractions. Force depression following muscle shortening for the voluntary contractions tested in this study did not depend on the distance or the speed of muscle shortening. These results are in contrast to the corresponding results in the literature obtained using artificial electrical stimulation in which force depression was always found to be directly related to the distance of shortening and inversely related to the speed of shortening. The difference in force depression as a function of the distance and speed of muscle shortening between voluntary and artificial electrical stimulation may be associated with changes in activation following the voluntary shortening contractions, whereas activation is controlled and constant in all artificial stimulation protocols.     

The effect of muscle fatigue on the isometric contractile characteristics of skeletal muscle in the cat

The rise time of an isometric twitch, the tetanic tension, the twitch tetanus ratio, the frequency-tension relationship, and the height of the MUAP (motor unit action potential) were measured in fast twitch (medial gastrocnemius) and slow twitch (soleus) muscles of the cat immediately before, in the middle, and immediately after fatiguing isometric contractions at tensions of 30, 50 and 80% of each muscle's initial strength (tetanic tension recorded from the unfatigued muscle). Although the twitch-tetanus ratio was always less for the soleus than for the medial gastrocnemius muscles, the twitch-tetanus ratio for any one muscle was constant throughout the duration of fatiguing isometric contractions at any of the tensions examined. In contrast, the twitch tension and tetanic tension of the muscles were both less after the contractions, the largest reduction occurring for both muscles during contractions sustained at the lowest isometric tensions. The time to peak tension of an isometric twitch was prolonged for both muscles following the contractions. This was associated with a corresponding shift in the frequency tension relationship such that at the point of muscular fatigue, the muscles tetanized at lower frequencies of stimulation than did the unfatigued muscle. In contrast, the amplitude of the MUAP showed only a modest reduction throughout the duration of the fatiguing contractions.     

The short-time Fourier transform and muscle fatigue assessment in dynamic contractions

The mean frequency of the power spectrum of an electromyographic signal is an accepted index for monitoring fatigue in static contractions. There is however, indication that it may be a useful index even in dynamic contractions in which muscle length and/or force may vary. The objective of this investigation was to explore this possibility. An examination of the effects of amplitude modulation on modeled electromyographic signals revealed that changes in variance created in this way do not sufficiently affect characteristic frequency data to obscure a trend with fatigue. This validated the contention that not all non-stationarities in signals necessarily manifest in power spectral parameters. While an investigation of the nature and effects of non-stationarities in real electromyographic signals produced from dynamic contractions indicated that a more complex model is warranted, the results also indicated that averaging associated with estimating spectral parameters with the short-time Fourier transform can control the effects of the more complex non-stationarities. Finally, a fatigue test involving dynamic contractions at a force level under 30% of peak voluntary dynamic range, validated that it was possible to track fatigue in dynamic contractions using a traditional short-time Fourier transform methodology.     

Muscle weakness following sustained and rhythmic isometric contractions in man

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.     

Fatigue and recovery contractile properties of young and elderly men

The 24 h recovery pattern of contractile properties of the triceps surae muscle, following a period of muscle fatigue, was compared in physically active young (25 years, n = 10) and elderly (66 years, n = 7) men. The fatigue test protocol consisted of 10 min of intermittent submaximal 20 Hz tetani. The maximal twitch (Pt) and tetanic force at 3 frequencies (10, 20 and 50 Hz) were determined at baseline and at 15 min, 1, 4 and 24 h after fatiguing the muscle. Maximal voluntary contraction (MVC) and vertical jump (MVJ) were also assessed. The loss of force during the fatigue test was not significantly different between the young (18 +/- 13%) and elderly (22 +/- 15%). Both groups showed similar and significant reductions of Pt (15%), tetanic force (10 to 35%) and rate of force development (dp/dt) (20%) 15 min and 1 h into recovery. The loss of force was greater at the lower stimulation frequencies of 10 and 20 Hz. Time-to-peak tension was unchanged from baseline during recovery in either group. The average rate of relaxation of twitch force (-dPt/dt) was decreased (p less than 0.05) and half-relaxation time significantly increased at 15 min and 1 h in the elderly but not the young. The findings indicate that after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels of the rate and time course of twitch relaxation compared to the young.     

Force-frequency relationship and potentiation in mammalian skeletal muscle.

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.